CN will re-ask for password after every workstation lock, also it can be configured to lock after a timeout or immediately after minimizing.Please post your settings from the File/Password protection dialog.

I think the 'Workstation lock' is the cause.Can I disable this behaviour?

The reason I use the password protection is, that I use the portable version and sync my database via dropbox. So encryption on disc is quite a reasonable step. But in my case I don't see a reason why a workstation lock should trigger a new 're-ask' dialog!?